When the pressure characteristics of the line pressure of a hydraulic pressure control apparatus are set in agreement with the driving state of an ordinary engine, pressure setting becomes too low for the reverse torque of the engine on operation of an exhaust brake, so that problems are raised in connection with durability characteristics of the friction engaging elements. In order to cope with such problems, a technique of elevating the line pressure to a predetermined pressure as a function of the exhaust braking signal has been proposed in the JP Patent KOKAI Publication No. 62-061843 (1987).